Many attempts have been made to simulate natural daylight by artificial means. It has been claimed, with some justification, that natural daylight is the preferred lighted environment. Thus, for example, in form 01128809L 150M (1990), the Duro-Test Corporation (of 9 Law Drive, Fairfield, N.J.) states that a good simulation of natural daylight ". . . encourages people to perform as never before because it promotes good vision . . . People see better and work better . . . " Thus, in form 0090 (1988), the Duro-Test Corporation states that light which ". . . simulates natural daylight . . ." is ". . . the perfect interior lighted environment. . . ."
One of the recognized experts on the use of color and its effects on people, Mr. Carlton Wagner, has published a book entitled "Beyond Beautiful" (The Wagner Institute for Color Research, Santa Barbara, Calif., 1990). At page 38 of this book, he recommends that, when women are evaluating the colors of objects, they should "arrange them . . . under a good light source (daylight or bright incandescent lighting is best . . . ").
The standard spectrum for daylight is referred to in D. L. MacAdam's "Color Measurement: Theme and Variations" (Springer-Verlag, N.Y., 1981.) In this book, the author refers to the CIE's D65 illuminant.
It is desirable to be able to simulate other daylight spectra, besides the D65 spectra. Thus, as is well known to those skilled in the art, the spectra of daylight will vary depending upon the daylight, upon atmospheric conditions, and solar altitude; see, e.g., S. T. Henderson's "Daylight and Its Spectrum," Second Edition (John Wiley & Sons, New York, 1977).
There are devices known to the prior art which are capable of generating different spectral distributions; see, e.g., the device disclosed in German patent 1,744,824 of Karl. However, as the spectra generated by these devices change, the intensity of such spectra changes.
For many applications, it is desirable that a sample be viewed under different spectral distributions at a substantially constant irradiance. Thus, for example, A.S.T.M. Standard D-1729-60T, entitled "Visual Evaluation of Color Differences in Opaque Materials," is often used for color matching of painted surfaces. The A.S.T.M. Standard requires that a specified opaque surface is to be viewed under different spectral conditions and that the irradiance of each of such condition be substantially constant. Thus, in section 5.1.2 of the Standard, it is provided that "For critical evaluation of color differences of materials of medium lightness, the illumination at the center of the viewed area shall be 100 to 125 foot-candles . . . In viewing very light materials, the illumination may be reduced to as low as 50 foot candles . . . and for the very dark materials may be increased to as much as 200 foot candles . . ."
The device of the Karl patent is not capable of producing different spectral distributions at a substantially constant level of irradiance. The prior art repeatedly suggests that, although different spectral distributions may be produced from a single light source and a multiplicity of filters, such distributions will have substantially different irradiances. Thus the "Kodak Color Darkroom Dataguide" (published by the Photographic Products Group, Eastman Kodak Company, Rochester, N.Y. 14650) discusses the adjustment one must make when developing prints using a single light source and different filters. At page 48 of this dataguide, in the paragraph entitled "Exposure Adjustments for Filter-Pack Changes," it is taught that "The overall density of a print is affected by changes in the number and density of the filters in the filter pack . . . For example, if you add 10M to the filter pack, increase exposure by 10 percent." The filter pack referred to in this quotation is mentioned at page 39 of the dataguide, in a paragraph entitled "Kodak Color Printing (CP) Filters" which states that "You place these acetate filters between the light source and the negative . . . When you change the filter pack, you can calculate adjustments from the filter factors for the filters you add or remove."
Not everyone who uses darkroom apparatus has both the expertise, the time, and the patience to calculate correction factors for the differing spectral distributions which must be used to correctly expose a photographic print. Not everyone who uses color matching apparatus realizes the importance of viewing the objects with spectra which have substantially constant irradiances.
In applicant's U.S. Pat. No. 5,079,683, an apparatus is provided which is able to automatically make the adjustments suggested in the Eastern Kodak dataguide. However, the device of this patent is relatively large, must contain a several fans to cool the optical components, and contains a a relatively large and expensive filter extrusion device. Furthermore, the device of this patent is not readily able to illuminate a large area. Because of these factors, this device also is relatively expensive to manufacture and its markets are limited.
It is an object of this invention to provide an apparatus which is as useful in every respect as the device disclosed in U.S. Pat. No. 5,079,683 (the disclosure of which is hereby incorporated by reference into this specification) but which, additionally, is less expensive to produce, less cumbersome, does not require as much cooling, and produces both more illumination and a broader area of illumination.